The present invention relates to the field of body volume measurement, body composition estimation (fat as a percentage of body weight) and nutritional assessment. It relies on the well-known relationship between body density and body composition.
The measurement of body composition is important in the management of acute and subacute illnesses. Excessive body fat is associated with, and can have an adverse effect on, a number of disease processes in adults, including cardiovascular disease, diabetes, hypertension, pulmonary diseases, and kidney disease.
A variety of techniques are in use today, including measurement of skinfolds, bioelectric impedance, total body electrical conductance, and determination of body density by hydrostatic weighing (using Archimedes principle). All of these techniques are plagued by significant problems.
Skinfold measurement suffers from population-specific prediction equations, variations in fat patterning, and a requirement for significant technician training.
Bioelectric impedance measurement can be affected by the state of hydration and the temperature of the subject and surrounding environment.
Measurement of total body electrical conductance requires very expensive equipment and which occupies a large amount of space.
Hydrostatic weighing requires immersion in a tank of water which is difficult or impossible for many individuals. In addition, the necessary measurement of residual lung volume requires expensive apparatus, including a nitrogen or helium analyzer.
In addition, there is no suitable method for the assessment of dietary needs in premature infants. A non-invasive estimation of body composition would provide neonatologists with an important tool that is not now available.
Various investigators have attempted to estimate body volume using plethysmographic techniques, but generally the results have fallen short of the precision required for useful estimation of body composition in terms of fat and lean body mass.
Gundlach, U.S. Pat. No. 4,369,652, has achieved fairly good results, but that apparatus requires that the subject be wrapped in a down or polyester "cocoon." In addition, elaborate precautions to control temperature are required.
Gundlach's apparatus uses parallel pistons to produce volume changes in two chambers, with the difference in the pressures of the two chambers being measured. In distinction to the present invention, elaborate means are required to produce precisely proportional volume changes in the two chambers. Also, air circulation between the two chambers is impossible with parallel volume perturbation.
Brachet U.S. Pat. No. 4,184,371 describes a two-chamber body system in which parallel pistons perturb the chambers with one piston of variable displacement so that pressure perturbations can be equalized. No provision is made to take into account the differences in compressability of air in the chamber and air in the lungs. Therefore, there is little expectation that the necessary precision could be achieved by this system.
Sheng U.S. Pat. No. 4,640,130 describes a system in which the frequency of a Helmholz resonator is varied by inclusion of a subject in the body of the resonator. As with Brachet, no attempt is made to account for the different behavior of air in the lungs, resulting in poor accuracy.
Fletcher U.S. Pat. No. 3,769,834 describes a single chamber device to assess body volume changes in astronauts. Cyclical volume perturbation induces pressure changes in the chamber which are related to the volume of the chamber minus the volume of the astronaut. Fletcher's method includes external respiratory connections. Because a large acoustic leak exists through tissue coupling from the chamber to the interior of the lungs and thence through the airways to the outside of the chamber, this system could not achieve the required accuracy. Additionally, the output of such a device would vary with barometric pressure.
A variety of inventions exist for the measurement of tank volumes, liquid level in a tank, and the volumes of irregular objects, and the volume of meat samples. (cf Doshi U.S. Pat. No. 4,704,902, Parker, U.S. Pat. No. 4,474,061, Nienisi U.S. Pat. No. 4,770,033, Turner U.S. Pat. No. 4,112,738, Leger U.S. Pat. No. 3,487,698, Hardway U.S. Pat. No. 3,113,448, Hamilton U.S. Pat. No. 3,129,585, Taylor U.S. Pat. No. 3,282,115, Thyron U.S. Pat. No. 4,713,966, Keng U.S. Pat. No. 3,585,861, Pond U.S. Pat. No. 4,561,298). These references generally suffer from a variety of defects that make them unadaptable for body volume measurement with the precision required for body composition assessment. In particular, none of them have any provision in apparatus or computation to take into account the different behavior of air in the lungs and in the chamber at large.
Only Gundlach of the above cited references has demonstrated an understanding of the need to deal with the fact that air in the lungs exists in substantially isothermal conditions, in distinction to the other air in the chamber.
But Gundlach requires a cumbersome arrangement of down or polyester fiber surrounding the subject, elaborate means to insure proportional volume perturbation, and precautions to insure very constant temperature conditions, as well as elaborate precautions to nullify other temperature effects.
Therefore, a need exists for a practical, accurate and noninvasive apparatus to estimate body volume and body composition.